linux-hardening - Re: [PATCH 62/64] netlink: Avoid false-positive memcpy() warning

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Date:   Thu, 29 Jul 2021 18:39:22 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-hardening@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Keith Packard <keithpac@...zon.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-staging@...ts.linux.dev, linux-block@...r.kernel.org,
        linux-kbuild@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH 62/64] netlink: Avoid false-positive memcpy() warning

On Wed, Jul 28, 2021 at 01:24:01PM +0200, Rasmus Villemoes wrote:
> On 28/07/2021 07.49, Greg Kroah-Hartman wrote:
> > On Tue, Jul 27, 2021 at 01:58:53PM -0700, Kees Cook wrote:
> >> In preparation for FORTIFY_SOURCE performing compile-time and run-time
> >> field bounds checking for memcpy(), memmove(), and memset(), avoid
> >> intentionally writing across neighboring fields.
> >>
> >> Add a flexible array member to mark the end of struct nlmsghdr, and
> >> split the memcpy() to avoid false positive memcpy() warning:
> >>
> >> memcpy: detected field-spanning write (size 32) of single field (size 16)
> >>
> >> Signed-off-by: Kees Cook <keescook@...omium.org>
> >> ---
> >>  include/uapi/linux/netlink.h | 1 +
> >>  net/netlink/af_netlink.c     | 4 +++-
> >>  2 files changed, 4 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
> >> index 4c0cde075c27..ddeaa748df5e 100644
> >> --- a/include/uapi/linux/netlink.h
> >> +++ b/include/uapi/linux/netlink.h
> >> @@ -47,6 +47,7 @@ struct nlmsghdr {
> >>  	__u16		nlmsg_flags;	/* Additional flags */
> >>  	__u32		nlmsg_seq;	/* Sequence number */
> >>  	__u32		nlmsg_pid;	/* Sending process port ID */
> >> +	__u8		contents[];
> > 
> > Is this ok to change a public, userspace visable, structure?
> 
> At least it should keep using a nlmsg_ prefix for consistency and reduce
> risk of collision with somebody having defined an object-like contents
> macro. But there's no guarantees in any case, of course.

Ah, good call. I've adjusted this and added a comment.

Thanks!

-Kees

-- 
Kees Cook